Probing interfacial electronic states in CdSe quantum dots using second harmonic generation spectroscopy

Understanding and rationally controlling the properties of nanomaterial surfaces is a rapidly expanding field of research due to the dramatic role they play on the optical and electronic properties vital to light harvesting, emitting, and detection technologies. This information is essential to the continued development of synthetic approaches designed to tailor interfaces for optimal nanomaterial-based device performance. In this work, closely spaced electronic excited states in model CdSe quantum dots (QDs) are resolved using second harmonic generation (SHG) spectroscopy, and the corresponding contributions from surface species to these states are assessed. Two distinct spectral features are observed in the SHG spectra, which are not readily identified in linear absorption and photoluminescence excitation spectra. These features include a weak band at 395 ± 6 nm, which coincides with transitions to the 2S_1/2 1S_e state, and a much more pronounced band at 423 ± 4 nm arising from electronic transitions to the 1P_3/2 1P_e state. Chemical modification of the QD surfaces through oxidation resulted in disappearance of the SHG band corresponding to the 1P_3/2 1P_e state, indicating prominent surface contributions. Signatures of deep trap states localized on the surfaces of the QDs are also observed. We further find that the SHG signal intensities depend strongly on the electronic states being probed and their relative surface contributions, thereby offering additional insight into the surface specificity of SHG signals from QDs.